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IEEE/CSC & ESAS NEWS FORUM (global edition), October 2017. MT25 Invited presentation Mo-Af-Or4-01.

1 JT-60SA Magnet System Status

S. Davis, W. Abdel Maksoud, P. Barabaschi, A. Cucchiaro, P. Decool, E. Di Pietro, G. Disset, N. Hajnal, K. Kizu, C. Mayri, K. Masaki, J.L. Marechal, H. Murakami, G.M. Polli, P. Rossi, V. Tomarchio, K. Tsuchiya, D. Tsuru, M. Verrecchia, M. Wanner

Abstract The JT-60SA experimental device will be the world’s is ramped from one extreme to the other current can be driven largest superconducting when it is assembled in 2019 in in the . However this technique limits the plasma dura- Naka, (R=3m, a=1.2m). It is being constructed jointly by institutions in the EU and Japan under the Broader Approach tion to one reversal of the solenoid current. agreement. For steady-state operation, desirable for power generation, Manufacturing of the six NbTi equilibrium field coils, which other non-inductive means to drive plasma current are being have a diameter of up to 12 m, has been completed. So far 13 of developed. These typically exploit other plasma heating tech- the 18 NbTi toroidal field coils, each 7 m high and 4.5 m wide, nologies such as the injection of high energy particles or reso- have also been manufactured and tested at 4 K in a dedicated test nant heating of the ions or the electrons by electromagnetic facility in . The first three of four Nb3Sn central solenoid modules have been completed, as have all of the copper in-vessel waves. error field correction coils. Also to this end when completed JT-60SA will be equipped Installation of the toroidal field magnet, around the previously with 34 MW of neutral beams (the particles must be neutral- welded 340° tokamak vacuum vessel and its thermal shield, start- ized once accelerated in order to penetrate the confining mag- ed at the end of 2016 and is currently underway. The TF magnet will in turn support the EF and CS coils. netic fields) and 7 MW of electron cyclotron resonance heat-  ing (gyrotron-generated broadcast into the plas- Index Terms—JT-60SA, tokamak, magnet, fusion ma). Localized and directional application of these techniques allows not only current drive but also control of the plasma current / / density profile. This can improve sta- I. INTRODUCTION bility, avoiding damage from disruptions, and allow improved he JT-60SA tokamak aims to contribute to the timely core conditions relative to the machine size (i.e. steeper gradi- Trealization of by complementing the burning ents). plasma research of ITER [1]-[3]. Its design fulfils its scientific The generation of one or more “pedestals” in the tempera- goals while being strongly optimized to reduce its cost. In par- ture and pressure profiles is dependent on high auxiliary heat- ticular the device aims to develop “advanced” plasma scenari- ing and is essential to achieve fusion-relevant conditions in an os relevant to achieving fusion conditions in a power plant, i.e. affordably-sized device. This allows access to the “H-mode” operational modes in which regime, with improved temperature confinement, and beyond. a) plasma current is driven by means other than induction Further stability control is provided by stabilizing plates and b) the ratio between plasma pressure and the confining to- by fast-acting actively controlled coils, both installed inside roidal field is high the primary vacuum vessel adjacent to the plasma. A pair of c) plasma conditions and plasma-wall interactions are copper poloidal coils maintain the vertical position of the achieved for durations relevant to steady-state opera- plasma while a set of 18 copper saddle coils distributed around tion. the react at approximately 3 kHz to limit the develop- In a classical tokamak current is induced in the plasma by ment of magnetohydrodynamic instabilities, primarily resis- the transformer effect, with the plasma forming a single sec- tive wall modes. ondary turn around the machine axis and a solenoid magnet making many primary turns. While the current in the solenoid II. MAGNET SYSTEM DESIGN AND MANUFACTURING

All the toroidal field (TF), equilibrium field (EF) and cen- S. Davis (email: [email protected]), P. Barabaschi, E. Di Pietro, N. Hajnal, V. Tomarchio, M. Verrecchia and M. Wanner are with Fusion for Ener- tral solenoid (CS) coils for JT-60SA are superconducting [4]- gy, Broader Approach Programme & Delivery Department, Boltzmannstr 2, [7]. This is necessary not only to achieve the fields required in 85748 Garching, Germany. a such a compact device but also to allow longer plasma dura- W. Abdel Maksoud, G. Disset and C. Mayri are with CEA-IRFU, 91191 Gif sur Yvette Cedex, France. tions and to avoid resistive power losses, both critical for fu- P. Decool and J.L. Marechal are with CEA-IRFM, 13108 St Paul lez sion power generation. Cedex, France. A. Cucchiaro, G.M. Polli and P. Rossi are with ENEA FSN, Via E. Fermi 45, Frascati, . A. Toroidal field coils K. Kizu, K. Masaki, H. Murakami, K. Tsuchiya and D. Tsuru are with QST, 801-1 Mukoyama, Naka-shi, Ibaraki-ken 311-0193, Japan. IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), October 2017. MT25 Invited presentation Mo-Af-Or4-01.

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Fig. 1: Equilibrium field coils and central solenoid Fig. 2: Toroidal field coil assembly The toroidal field coil assembly (Fig. 2) forms the structural A series of slight shape changes were observed during the backbone of the magnet system [8]. Only net forces acting on manufacturing of the TF coils. After being released from their the magnets, e.g. the weight of the assembly and loads from tooling after winding and vacuum impregnation the coils were earthquakes or plasma disruptions, must be supported exter- typically slightly ‘bean-shaped’ rather than D-shaped, with the nally. This function is achieved by the “gravity supports” at middle of both the straight leg and the curved leg bowing in- the bottom of each TF coil which pivot on spherical bearings wards by a few millimeters. The welding of the casings caused to allow radial movement when the magnets are cooled down a deformation in the opposite direction, with the coils legs to cryogenic . bowing outwards by a few millimeters with respect to nominal There are 18 D-shaped toroidal field coils [9] surrounding to make the coils more round. Hence it was possible to im- the torus-shaped vacuum vessel in which the plasma is gener- prove the final shape of the winding packs within the casings ated. At the inboard side their straight legs wedge against each by wedging them carefully before welding to anticipate this other while at the outboard side their curved legs are free to effect. move radially. They are supported against the toroidal forces (normal to the plane of the each coil) generated by the interac- B. Equilibrium field coils tion of their own field and the fields from the plasma and po- loidal field coils by the Outer Intercoil Structure (OIS) and the There are 6 equilibrium field coils and 4 independently- Inner Intercoil Structures (IIS). The OIS allows each coil to be controlled solenoid modules (Fig. 2). These coils allow wide bolted to its neighbors through shear panels while the IIS in- ranging and flexible control of the plasma shape. Plasma as- cludes force fit shear pins between neighboring coils at their pect ratios (major radius / minor radius) down to 2.5 will be top and bottom. possible, allowing efficient exploitation of the available toroi- Each coil has 72 turns wound from NbTi cable-in-conduit dal field. Furthermore both double and single null configura- conductor and arranged in 6 double pancakes, forming a com- tions can be created, i.e. the poloidal field can be configured to pact winding pack with a rectangular cross section. A steady 25.7 kA current produces a nominal 2.25 T field on the toka- mak axis (max 5.65 T on the conductor). The toroidal field magnet has a stored energy of 1.06 GJ. The toroidal field coils are manufactured in Europe [10]- [16] and shipped after testing to Japan. All of the winding packs needed have been wound, including the electrical joints between double pancakes, application of ground insulation and vacuum impregnation. The last coils are now being inte- grated with their stainless steel casings. After the casing has been welded closed a final epoxy resin impregnation is per- formed to embed the winding packs inside. Then the mechani- cal interfaces on the coil casing are machined with respect to the average winding pack centreline and the piping, temperature sensors and quench detection cables are fitted. Fig. 3: EF coils 1-3 IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), October 2017. MT25 Invited presentation Mo-Af-Or4-01.

3 form a “” or plasma exhaust at both the top and bottom lation of the final module is beginning. Once completed the of the torus. This allows the removal of impurities, maintain- four modules must be stacked and compressed with tie rods. ing core conditions, while managing the high heat flux from The finished solenoid will weigh almost 100 tonnes. the exhaust particles. Within the available flux swing of 17 Vs JT-60SA aims to The EF coils vary in diameter from 4.4 m up to 12 m. Two drive a plasma current of up to 5.5 MA inductively for 100 s. different NbTi cable-in-conduit conductors are used with a Non-inductively a current of 2.3 MA is targeted. central spiral coolant channel, one optimized for a peak field of 4.8 T for EF coils 1, 2, 5 & 6 and one optimized for a peak field of 6.2 T for the inboard EF coils 3 and 4. Both carry up to 20 kA. Two pick-up coils are co-wound in each coil for quench protection. Manufacturing of the EF coils [17]-[20] was completed in 2016. Since their large size (up to 12 m diameter for EF1) prohibits their transportation most were manufactured on site in Naka using modularized tooling. The coils are wound from single or double pancakes, depending on their size and hence how many turns can be made from a single conductor length. The pancakes are press-cured clamped in a mould before they are stacked to form the winding pack. Then the ground insula- tion is wrapped and cured before clamps are fitted around the winding packs to keep them under compression during opera- tion and provide support points. Despite their large size excellent control of deviations from circularity was achieved during the winding and stacking of

TABLE 1 EF COIL CIRCULARITY

Deviation from Coil Diameter Requirement circularity EF1 12.0 m 0.3 mm 8 mm EF2 9.6 m0.4 mm 7 mm EF3 4.4 m0.2 mm 6 mm EF4 4.4 m0.6 mm 6 mm EF5 8.1 m0.6 mm 7 mm EF6 10.5 m 1.3 mm 8 mm

Fig. 4: Central solenoid module 1 the EF coils, as shown in Table 1. Continuous improvement allowed lower values to be achieved for EF 1-3 (Fig. 3) than for EF 4-6. III. ELECTRICAL TESTING C. Central solenoid The performance of the superconducting strand and conduc- tors used in the manufacturing of the coils was demonstrated The central solenoid [20]-[24] is made of 4 modules, each previously during both qualification tests and routine quality with an outer diameter of 1.65 m and a height of 1.60 m. Each assurance [25]-[30]. module is made from 6 octa-pancakes and 1 quad-pancake forming 549 turns. The conductor can carry 20 kA, giving a total of almost 11 MA-turns for each module. A. Toroidal field coils The peak nominal field of 8.9 T necessitates a Nb3Sn c ca- All of the TF coils for JT-60SA are tested under Paschen ble-in-conduit conductor and hence the pancakes must be conditions by their manufacturers, i.e. their ground insulation wound before the 650°C heat treatment needed to produce the is tested at 3.8 kV while they are surrounded by gas at a range superconducting strand is performed on the octa / quad pan- of low pressures at which a Paschen breakdown could occur. cakes. After heat treatment the turns must be separated care- Typically faults observed have been limited to lengths of the fully, without applying excessive strain, to allow the glass- helium piping or joints which were insulated by hand. These kapton turn insulation to be wrapped around the conductor. were corrected and re-tested. Finally the pancakes are impregnated together to form each of All of the TF coils are also tested under cryogenic condi- the four modules. tions at their nominal current and all of them will be quenched As of August 2017 three of the four solenoid modules have in order to demonstrate they have sufficient operational mar- been completed (the first may be seen in Fig. 4) and the insu- gin with respect to the and temperatures pre- IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), October 2017. MT25 Invited presentation Mo-Af-Or4-01.

4 sent. As of August 2017 13 of the 18 coils had successfully the plasma vacuum vessel; EF coils 1-3 and the CS will be in- completed these tests at a dedicated facility established at stalled once the TF magnet assembly is complete. CEA Saclay. Each quench occurred with a helium inlet temperature of A. TF coil pre-assembly between 7.44 and 7.51K. The quench was initiated in a central pancake, where the field is highest, for only one third of the After its cryogenic testing and before its shipment to Japan coils. For half of the coils the quench was initiated in a side each TF coil is pre-assembled together with a sector of the pancake. Outer Intercoil Structure. For this operation the coil is oriented The casings of the TF coils are not strongly cooled, given ‘on edge’, i.e. with its straight leg along the ground and its that the priority is to cool the winding packs and that there is curved leg up in the air. The U-sectioned OIS is lowered over very limited space for piping, especially on the straight legs. the coil from above (Fig. 5) and aligned with the symmetry During testing the casings were observed to remain between 5 plane of the winding pack. Openings in the coil casing allow and 15 K warmer than the winding packs, depending on the tracking points on the winding pack to be measured directly location. This is expected to be lower in the tokamak when the by laser tracker. Steel pads between the OIS and the coil are coil casings are surrounded by other structures cooled to 4 K adjusted to give a 0.5 mm gap. This allows the coil to move rather than the 70 K thermal shield of the test . radially during cooldown and when energized but to be sup- The hydraulic performance of each TF conductor was ported against out-of-plane loads. After the OIS has been fit- measured before it was wound and those conductors having ted each coil is returned to a horizontal orientation, or rather below average performance were assigned to the intermediate 10° from horizontal given the wedged shape of the OIS, for double pancakes 2 and 5, i.e. away from the highest field and shipping to Japan. highest temperatures. The pressure drop of each coil is con- firmed during the cryogenic tests. All internal joint resistances were measured, with the max- imum resistance measured for an individual joint being 1.57 nΩ. The joints are equipped with temporary voltage taps which are removed before installation in the tokamak.

B. Equilibrium field coils Due to their very large size the production EF coils are not tested under Paschen or cryogenic conditions. The integrity of their ground insulation is demonstrated by spraying the coils with water and covering them with aluminum foil before the 21 kV test voltage is applied. Fig. 5: TF coil pre-assembly with Outer Intercoil Structure

C. Central solenoid B. TF coil positioning For the central solenoid, a test coil [31] comprising one quad pancake and one double pancake was wound and tested On arrival on site at QST Naka a number of tests are per- at 4 K. This demonstrated the hydraulic performance of the formed on the coils to ensure they are free from transportation conductor and the resistance of the pancake joints and the ter- damage, e.g. pressure and leak tests, ground insulation test etc. minal joints used. The coils must be assembled around the vacuum vessel and its thermal shield already welded together. These form an incom- Limited cryogenic tests were also performed on the first plete torus leaving a 20 degree slice open to allow for the in- completed CS module, which indicated that the 0.75 K heating sertion of the TF coils. The tokamak assembly is surrounded expected from AC losses occurring during a fast discharge by a temporary assembly frame capped by a rotary crane that should not exceed the 1 K temperature margin. These tests al- allows each coil, once inserted, to be rotated into its final posi- so indicated that the temperature uniformity during cooldown tion. and the cooldown rate were expected to be sufficient. First each coil is stood upright using a dedicated frame. Then it is lifted to the center of the tokamak using the building crane and a lifting beam to ensure the coil stays level during the lift. The coil is then lowered between the two parallel IV. ASSEMBLY STATUS beams of the rotary crane into the 20° opening in the vacuum vessel. Here it is supported temporarily while it is transferred The assembly of the toroidal field coils is currently under- from the building crane to the rotary crane. While being way and as of August 2017 eleven coils have been accurately brought to its target position the coil is shifted slightly out- positioned in the torus hall. EF coils 4-6 were positioned be- wards radially to increase the clearance between the straight forehand on the cryostat base in 2013 before the assembly of leg and the vacuum vessel thermal shield. The temporary sup- IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), October 2017. MT25 Invited presentation Mo-Af-Or4-01.

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TABLE 2 TF COIL POSITIONS ACHIEVED – VARIATIONS FROM NOMINAL LOCAL CO-ORDINATES (IN MM)

Casing tracking point B A G H C x 0.1 -0.2 -1 2.3 1.1 Coil position 3 y 0.1 0.4 0 0.5 0.6 (11th installed, no. 6) ｚ 0.8 0.9 0.5 1.4 1.6 x 0.1 -0.1 -1.0 2.0 0.7 Coil position 4 y -0.4 0.0 -0.3 0.7 0.5 (9th installed, no. 5) ｚ -0.1 0.1 0.1 0.6 0.9 x 0.4 -0.6 -1.3 1.3 0.3 Coil position 5 y -0.6 -0.5 -0.1 -0.3 -0.3 (7th installed, no. 14) ｚ 0.3 0.2 0.1 1.5 1.6 x 0.8 0.7 -0.3 2.4 1.2 Coil position 6 y -0.2 0.8 0.3 0.1 0.0 (4th installed, no. 3) ｚ 0.4 0.7 0.6 1.5 1.8 x 0.0 0.3 -0.4 1.8 0.3 Coil position 7 y -0.4 0.4 -0.1 -0.4 0.3 (3rd installed, no. 1) ｚ 0.1 0.5 0.3 0.5 0.9 x 0.5 0.5 -0.9 2.8 1.1 Coil position 8 y 0.0 0.6 0.0 -0.3 -0.4 (1st installed, no. 10) ｚ -0.4 0.5 0.1 0.9 1.3 Fig. 6: Main TF coil casing points tracked during assembly x 0.2 0.4 -1.0 2.0 0.6 Coil position 9 y 0.1 0.2 0.0 0.2 -0.2 (2nd installed, no. 11) ports of the vacuum vessel must be dismounted and reinstalled ｚ 0.2 0.2 0.1 0.5 0.6 one by one as the coil is rotated around it. Laser levels are x 0.3 1.1 -0.8 2.8 1.1 Coil position 10 y 0.0 0.6 -0.1 -0.4 -0.2 used to ensure that the vertical positon of the vacuum vessel is (5th installed, no. 12) maintained. ｚ 0.2 -0.4 0.5 0.8 1.0 x 0.3 0.2 -1.2 2.1 0.8 Coil position 11 The positioning of each coil is checked using tracking y 0.3 0.3 0.1 1.0 0.2 (6th installed, no. 13) points machined on the outside of its casing at known posi- ｚ 1.0 0.4 0.5 1.3 1.7 tions with respect to the winding pack inside. The primary x 0.3 0.6 -0.2 2.1 0.8 Coil position 12 y 0.2 -0.1 0.2 -0.6 0.8 points tracked (Fig. 6) are A and B at the top and bottom of (8th installed, no. 4) the straight leg, ensuring its radial position (x direction) and ｚ 0.5 0.7 0.6 1.4 1.3 x -0.1 0.1 -1.3 2.8 1.4 Coil position 13 verticality (z), the middle of the straight leg G, to ensure the y 0.6 0.4 -0.5 0.2 -0.1 (10th installed, no. 15) coils correct vertical position, and the middle of the curved leg ｚ -0.9 0.5 0.8 1.0 1.1 H, ensuring its correct toroidal positioning (y). Three supports are used to position each coil. At the bottom of each straight leg an accurately positioned jig is used to lo- with the aim of ensuring contact where the coils are bolted at cate the crown plates of the coil, setting both its vertical posi- their crown plates at the top and bottom of the straight leg and tion and radial position. At the bottom of the curved leg, at the of ensuring a gap of below 0.5 mm along the straight leg. position from which the EF5 coil will eventually be supported, double-acting hydraulic jacks allow the outboard of the coil to D. Inner Intercoil Structures be raised or lowered. A little higher up the curved leg near point C screws may be used to adjust the position of the EF6 The Inner Intercoil Structures (Fig. 7) are a set of bolts and pedestal in the horizontal plane, i.e. toroidally (y) or radially pins fitted between neighboring coils at the top and bottom of (x). The coil is adjusted such that each tracking point is within 1 mm of its target position as shown in Table 2. Local co- ordinate systems centered on the tokamak vertical axis but aligned with the symmetry plane of each coil allow an easier assessment of the live coil position. The slender coils deform appreciably under their own weight – this may be observed in particular in the radial (x) movement of points G and C at the middle of the straight and the curved legs which bow inwards and outwards respectively.

C. Straight leg insulation To prevent toroidal eddy current circulation, each coil must be insulated from its neighbors and therefore before installa- tion each straight leg is fitted with 2 mm GRP plates on one side. The other side is fitted with stainless steel shims chosen Fig. 7: Lower inner intercoil structures IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), October 2017. MT25 Invited presentation Mo-Af-Or4-01.

6 each straight leg which from their manufacturing, pre-assembly with the TF coils and a) hold the coils together under the toroidal field , which final positioning in the torus. For each joint between adjacent generally pinches the straight legs of the coils together coils the installed position of each shear panel is measured at but also expands them under hoop tension. This acts to three bolt hole centers using a laser tracker. These data are separate adjacent crown plates, and used to design and then machine each splice plate accordingly. b) withstand the torque arising from the crossing of the TF Both the shear panels and, after machining, the splice plates with the poloidal field from the plasma and EF coils. are sandblasted to ensure consistent friction for the joint. GRP This causes the crown plates to try and shear against plates are fitted between the shear panels and the splice plates each other. to provide electrical insulation. The M30 and M36 studs and nuts are made in Inconel 718 Each pair of adjacent shear panels is sandwiched between a to withstand the high tensile and bending loads applied during pair of splice plates using 22 M42 Inconel 718 studs. These tokamak operation. 20 mm thick titanium washers are used to are all tensioned simultaneously using a set of 22 hydraulic help reduce the loss of preload that occurs due to the reduced tensioners. Simultaneous tensioning minimizes the loss of pre- thermal contraction of Inconel compared with the stainless load that occurs when load is transferred from the tensioners steel crown plates, and to allow larger bolt holes to accommo- to the nuts. date potential misalignment between coils. The threads are lu- As of August 2017 8 sets of splice plates have been custom- bricated with Loctite LB-8012 molybdenum paste and the nuts ized and delivered just-in-time to Naka. These may be seen fit- are tightened using hydraulic wrenches. The extension of the ted to the TF coils in Fig. 8. bolts is monitored using an ultrasonic extensometer to ensure the correct pre-load is achieved. Each pin assembly is made up of a conical Inconel pin and two split bushings. The bushings are made eccentric, i.e. with the axis of their inner conical surface offset from the axis of their outer cylindrical surface. By rotating the bushings in the crown plates, the axes of the conical surfaces may be brought into alignment even if the coils themselves are not perfectly aligned. Allowance is made for the axis of each crown plate pin hole to vary from its nominal position by up to 3 mm in any direction after manufacturing and installation, although the actual misalignments observed are much lower. The pins, which have a plasma-sprayed alumina coating to maintain the electrical separation of neighboring coils, are wedged into place using a hydraulic tensioner.

Fig. 8: Splice plates fitted between adjacent sectors of the OIS E. Gravity supports Each of the 18 TF coil gravity supports is pre-positioned on the cryostat base just prior to its respective coil. The main pin V. ERROR FIELD CORRECTION COILS between the coil casing and the upper spherical joint of the Besides the main superconducting coils described above gravity support is fitted after the position of the coil is final- and the fast plasma position control coils and the resistive wall ized. Finally shims are inserted under the two feet of the gravi- mode stabilizing coils mentioned in the introduction, JT-60SA ty support and they are bolted to their support pedestals on the will have a set of 18 saddle-shaped error field correction coils cryostat base. However the weight of the magnet will not be [32],[33] installed close to the plasma within the vacuum ves- transferred to these supports until the complete assembly is sel. These are intended to compensate for any small error in finished. the main field generated by TF and EF coils. Such compensa- tion is beneficial towards the containment of non- F. Splice plates axisymmetric plasma modes. The manufacturing of these coils On each side of each sector of the OIS there are 5 shear was completed in March 2017. panels which must be bolted through pairs of splice plates to those of the neighboring OIS sector. The complete Outer In- VI. CONCLUSION tercoil Structure is hence able to support the TF coils against toroidal loads. The space between shear panels was maxim- The manufacturing of the equilibrium field coils and the er- ized to allow large vacuum vessel ports and hence easy access ror field correction coils for JT-60SA is complete, and the for plasma diagnostics and heating systems. manufacturing of the toroidal field coils and the central sole- Each splice plate must be customized individually in order noid is well advanced. The assembly of the TF magnet is un- to accommodate the variations in shear panel position arising derway and will be completed in the first half of 2018. IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), October 2017. MT25 Invited presentation Mo-Af-Or4-01.

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